Fix bug introduced in rewrite of keg_free_slab in -r251894.

Fix bug introduced in rewrite of keg_free_slab in -r251894.
The consequence of the bug is that fini calls are not done
when a slab is freed by a call-back from the page daemon.
It went unnoticed for two months because fini is little used.

I spotted the bug while reading the code to learn how it works
so I could write it up for the next edition of the Design and
Implementation of FreeBSD book.

No MFC needed as this code exists only in HEAD.

Reviewed by: kib, jeff
Tested by:   pho